Effects of SOS and MucAB functions on reactivation and mutagenesis of M13 replicative form DNA bearing bulky lesions

The aflatoxin B(1) formamidopyrimidine adduct plays a major role in causing the types of mutations observed in human hepatocellular carcinoma

A G to T mutation has been observed at the third position of codon 249 of the p53 tumor-suppressor gene in over 50% of the hepatocellular carcinoma cases associated with high exposure to aflatoxin B(1) (AFB(1)). Hypotheses have been put forth that AFB(1), in concert with hepatitis B virus (HBV), may play a role in the formation of, and/or the selection for, this mutation. The primary DNA adduct of AFB(1) is 8,9-dihydro-8-(N(7)-guanyl)-9-hydroxyaflatoxin B(1) (AFB(1)-N7-Gua), which is converted naturally to two secondary lesions, an apurinic site and an AFB(1)-formamidopyrimidine (AFB(1)-FAPY) adduct. AFB(1)-FAPY is detected at near maximal levels in rat DNA days to weeks after AFB(1) exposure, underscoring its high persistence in vivo. The present study reveals two striking properties of this DNA adduct: (i) AFB(1)-FAPY was found to cause a G to T mutation frequency in Escherichia coli approximately 6 times higher than that of AFB(1)-N7-Gua, and (ii) one proposed rotamer of AFB(1)-FAPY is a block to replication, even when the efficient bypass polymerase MucAB is used by the cell. Taken together, these characteristics make the FAPY adduct the prime candidate for both the genotoxicity of aflatoxin, because mammalian cells also have similar bypass mechanisms for combating DNA damage, and the mutagenicity that ultimately may lead to liver cancer.

UV irradiation of Escherichia coli modulates mutagenesis at a site-specific ethenocytosine residue on M13 DNA. Evidence for an inducible recA-independent effect

Mutagenic action of chemical and physical mutagens is mediated through DNA damage and subsequent misreplication at sites of unrepaired damage. Most DNA damage is noninstructive in the sense that the causative chemical modification either destroys the template information or renders it inaccessible to the DNA polymerase. Noninstructive adducts possess high genotoxicity because they stop DNA replication. Replication past noninstructive adducts is thought to depend on induced functions in addition to the regular replication machinery. In Escherichia coli, noninstructive DNA damage leads to induction of the SOS regulon, which in turn is thought to provide the inducible functions required for replicative bypass of the lesion. Because of the absence of accessible template instruction, base incorporation opposite noninstructive lesions is inherently error-prone and results in mutagenesis. Ethenocytosine (epsilon C), an exocyclic DNA lesion induced by carcinogens such as vinyl chloride and urethane, is a highly mutagenic, noninstructive lesion on the basis of its template characteristics in vivo and in vitro. However, mutagenesis at epsilon C does not require SOS functions, as evidenced by efficient mutagenesis in recA-deleted E. coli. Even though efficient mutagenesis in recA-deleted cells shows a lack of SOS dependence, the question remains whether SOS induction can modulate mutagenesis opposite epsilon C. To examine the possible contribution of SOS functions to mutagenesis at epsilon C, we constructed an M13 duplex circular DNA molecule containing an epsilon C residue at a unique site. The construct was transfected into nonirradiated or UV-irradiated E. coli.(ABSTRACT TRUNCATED AT 250 WORDS)

Germline selection: population genetic aspects of the sexual/asexual life cycle

Population geneticists make a distinction between sexual and asexual organisms depending on whether individuals inherit genes from one or two parents. When individual genes are considered, this distinction becomes less satisfactory for multicellular sexual organisms. Individual genes pass through numerous asexual mitotic cell divisions in the germline prior to meiosis and sexual recombination. The processes of mitotic mutation, mitotic crossing over, and mitotic gene conversion create genotypic diversity between diploid cells in the germline. Genes expressed in the germline whose products affect cell viability (such as many "housekeeping" enzymes) may be subjected to natural selection acting on this variability resulting in a non-Mendelian output of gametes. Such genes will be governed by the population genetics of the sexual/asexual life cycle rather than the conventional sexual/Mendelian life cycle. A model is developed to investigate some properties of the sexual/asexual life cycle. When appropriate parameter values were included in the model, it was found that mutation rates per locus per gamete may vary by a factor of up to 100 if selection acts in the germline. Sexual/asexual populations appear able to evolve to a genotype of higher fitness despite intervening genotypes of lower fitness, reducing the problems of underdominance and Wright's adaptive landscape encountered by purely sexual populations. As might be expected this ability is chiefly determined by the number of asexual mitotic cell divisions within the germline. The evolutionary consequences of "housekeeping" loci being governed by the dynamics of the sexual/asexual life cycle are considered.

Genetic requirements for frameshift reversion induced by bulky DNA adducts in M13 DNA

In order to analyze the genetic requirements and mechanisms of frameshift mutagenesis by activated aflatoxin B1 (AFB1), in vitro-modified phage M13 replicative form (RF) DNA was transfected into appropriate Escherichia coli cells and +1 or -1 frameshift revertants in the lacZ(alpha) gene were isolated. This analysis shows that both +1 and -1 frameshift mutagenesis by AFB1 is significantly reduced in a umuC- background. On the other hand, in the absence of RecA, +1 frameshift mutagenesis is partially reduced, but -1 frameshift mutagenesis is unaffected. DNA sequence analysis of +1 frameshifts induced by AFB1 in recA- cells suggests that the mutations occur at the same sites as in recA+ cells, but that there are significant differences in the specificity of the observed base changes. A model consistent with the observed effects in the absence of RecA suggests that an appreciable fraction of AFB1-adducted guanines can correctly template for a cytosine.

Epoxides: comparison of the induction of SOS repair in Escherichia coli PQ37 and the bacterial mutagenicity in the Ames test

The genotoxicity of 51 epoxides is studied with the SOS-Chromotest using Escherichia coli PQ37 as tester strain. The results obtained with this test system are compared with results of the Ames test. Out of 51 epoxides, 39 are shown to be mutagenic in Salmonella typhimurium whereas only 27 mutagenic epoxides induced the SOS response in Escherichia coli PQ37.

Mutagenesis by aflatoxin in M13 DNA: base-substitution mechanisms and the origin of strand bias

The two goals of the experiments described here are: (a) to examine whether there is a strand bias in mutagenic processing of bulky lesions in M13 replicative form (RF) DNA, and (b) to examine the mutational mechanisms of metabolically activated aflatoxin. For these experiments, two types of nicked heteroduplex M13 RF DNA molecules (+WT/-am1 and +am1/-WT) in which either the minus (-) or the plus (+) strand carried a gene 1 amber nonsense codon, were constructed. Heteroduplex DNAs were modified in vitro with aflatoxin B1 activated by hamster liver S9 enzymes, and transfected into SOS(UV)-induced Escherichia coli (Supo/uvrA-/mucAB+). Forward mutations in the lacZ alpha-complementing gene segment were scored and sequenced. Results indicated that aflatoxin-induced mutation frequencies in the +WT/-am1 heteroduplex were significantly greater than those in the +am1/-WT heteroduplex, suggesting more efficient mutagenic processing of lesions in the plus strand. These results permit specific suggestions for improved mutation detection in the extensively used M13 forward mutagenesis system. Sequence analysis of point mutations from the +WT/-am1 experiments showed that most substitutions were targeted to plus-strand guanines. Both G-to-A transitions and G-to-T transversions were induced with equal efficiency. Since activated aflatoxin B1 is known to react almost exclusively with DNA guanines at the N7 position, these results suggest that bulky lesions at guanine N7 position may have the properties of mis-instructional as well as non-instructional lesions.